Carbon materials have the potential to be excellent absorbing materials due to their lighter weight, easy-to-control graphitization degree and good dielectric properties. However, a single carbon material is often difficult to achieve the ideal electromagnetic wave attenuation effect due to the imbalance of impedance matching and the lack of magnetic loss. In this work, a simple solgel method was used to control the hydrolysis reaction of tetraethyl orthosilicate and the synthesis reaction of phenolic resin to prepare the core-shell precursor of silica coated with phenolic resin/silica spheres. After high-temperature calcination and pickling etching, hollow carbon spheres (HPC) with porous structure were obtained. By adjusting the calcination temperature, the crystallinity and dielectric properties of HPC can be easily adjusted. With an ultra-low filling ratio of 5 wt%, HPC can obtain a wide-band effective microwave absorbing performance of 5.73 GHz (2-18 GHz band) under an ultra-thin thickness of 2.0 mm, which is an excellent absorbing agent with lightweight, thin thickness, strong absorbing ability and wide absorbing band. Considering the simple preparation scheme, HPC is expected to become one of the candidates for high-efficiency microwave absorbers. The special hollow, porous structure and suitable degree of graphitization are the main reasons for the effective wave absorption of HPC.